Binders or binder systems for foundry cores and molds are known. Generally, such binders should exhibit good thermal and dimensional stability in order to result in good dimensionally accurate metal castings. In addition, such binder systems should have rapid curing times and exhibit uniform curing properties, that is, the centers of cores or molds made therewith should be as well cured and as strong as their surfaces in order to minimize breakage or warpage.
In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or patterns and then cured with the use of catalysts and/or heat to a solid, cured state.
While many of the known binders or binder systems prepared with aggregate material, such as sand, and a binding amount of a binder, such as polymerizable or curable material, possess the required properties mentioned above and are suitable for use in the foundry industry, even further improvements in such materials are required so that they exhibit even better properties and readily lend themselves to suitable core or mold making processes.
In this respect, there have been developed in the foundry industry, a variety of different processes for forming molds or cores, the particular process employed being dependent upon the binder or binder system being utilized. For example, many liquid binder systems require that curing and hardening be accomplished in a holding pattern or core box while subjected to heat. An example of this type of process is the commonly known "hot box" process. An exemplary process of this type is disclosed in U.S. Pat. No. 3,306,864.
On the other hand, some binder systems, such as for example the phenol-formaldehyde benzylic ether resin systems, do not require heating. Such systems are utilized in processes, commonly referred to as "cold box" processes which are accomplished by passing gaseous catalyst through molded resin coated sand at ambient temperatures in order to achieve curing. In such systems, the resinous material is generally dissolved in a solvent and the type of solvent used affects curing speed and tensile strength. "Cold box" binders and processes as well as solvents employed therein are disclosed in U.S. Pat. No. 3,905,934 and 3,409,579.
Still other types of binder systems do not require gassing or heating in order to bring about curing or hardening.
Such systems are known as "no-bake" binders. Typical "no-bake" polyurethane binding systems of this type are disclosed in U.S. Pat. Nos. 3,499,861, 3,676,392 and 3,686,106. While these types of systems do not require gassing, many exemplary "no-bake" resinous systems, on the other hand, still require relatively long time periods to accomplish substantially complete curing at ambient temperatures or even under some heat.
Furthermore, although developments in resinous binder systems which can be processed according to the "cold box" process have resulted in the provision of useful systems based on phenol-formaldehyde benzylic ether resins and isocyanates which are employed with various solvents, the selection of which solvents materially affects curing speed and tensile strength, as disclosed in the above-mentioned U.S. Pat. Nos. 3,905,934 and 3,409,579, such systems still exhibit certain disadvantages. For example, while such systems are particularly useful in the foundry art in making cores or molds, they are somewhat deficient in that the molds or cores made therewith exhibit relatively low hot strength which often results in casting defects such as, "burn in" and erosion during use. There still exists, therefore, a need for the development of resinous systems which exhibit even more improved curing speeds, thus resulting in decreased production time per unit and, consequently, increased productivity, as well as providing molds therewith which have increased hot strength and, as well, elimination of casting defects, such as "burn in" and erosion. Moreover, there also exists a need for binder systems which readily lend themselves to processing to a molded, cured state by either "cold box" or "no-bake" processes. The present invention fulfills such needs.